1. Field of the Invention
This invention relates to a silicon semiconductor substrate and a method for the production thereof. More particularly, the invention relates to a silicon semiconductor substrate which originates in a silicon semiconductor substrate so shaped as to permit easy extinction of void type defects by a heat treatment with a view to obtaining a defect-free region in a void type product and which, in consequence of a subsequent heat treatment, forms a deep defect-free surface layer, excels in device characteristics, and enjoys a satisfactory gettering property and to a method for the production thereof.
2. The Prior Art
Heretofore, as regards the improvement of the defect-freeness of the surface layer of the semiconductor substrate, the technique of heat-treating a given semiconductor substrate in an atmosphere of hydrogen gas at a temperature of 1200xc2x0 C. for a duration of not less than one hour thereby inducing expansion of a defect-free layer devoid of oxygen precipitate defects to a depth of 10 xcexcm from the surface layer has been reported (JP-A-06-252,154). This technique has been known to effect the extinction of void type defects (i.e. xe2x80x9cempty holexe2x80x9d type defects) to a depth of 1 to 3 xcexcm. Recently, the technique of effecting the extinction of the void type defects to a deeper region from the surface layer by decreasing the void type defects in size at a high density in consequence of the addition of nitrogen has been reported (JP-A-11-135,511 and JP-A-2000-256,092). In the latter invention, it is reported that the change in form of void type defects attracted interest and the addition of nitrogen in a pertinent shape of the defects proved effective.
The prior art mentioned above has barely unveiled the effect of the addition of nitrogen on the change of form manifested in the void type defects. Regarding the heat treatment which is performed for the purpose of effecting extinction of the void type defects, it has not imposed any limitation on the effective nitrogen concentration, the oxygen concentration, and the cooling rate to be used while the silicon single crystal being pulled passes through a temperature zone of 1100xc2x0 C. (hereinafter referred to simply as xe2x80x9ccooling ratexe2x80x9d). Specifically, in the void type defects, the voids which are point defects are diffused through the surface of the void type defects during the extinction thereof by the heat treatment. The diffusion in this case is proportional to the surface area forming the peripheral parts of the void type defects. The mere mention of the effect in the change of form does not deserve to be deemed as imposing a necessary limitation on the extinction of void type defects.
In fact, the heat treatment contemplated by the prior art induces extinction of the defects barely to the extent of recording a residual ratio on the order of percent at a depth of 0.5 xcexcm from the surface layer. It has not imposed on the void type defects a limitation enough for effecting extinction of the void type defects to such a density that the manufacture of a device under production conditions excelling in commercial productivity will not be adversely affected.
Owing to the current demand to produce silicon single crystals in increased diameters, the void type defects possibly gain unduly in growth, depending on the relevant production conditions. In the light of this fact, the prior art is suspected of manifesting its effect only insufficiently unless the addition of nitrogen is made in an adequate amount. From the viewpoint of permitting manufacture of a commercially useful silicon semiconductor substrate enjoying extinction of void type defects throughout to a deep region from the surface layer, therefore, the prior art is at a disadvantage in lacking limitations of conditions.
It is, therefore, an object of this invention to provide a novel silicon semiconductor substrate and a method for the production thereof in view of such drawbacks of the prior art as mentioned above.
To be specific, this invention is capable of effecting extinction of void type defects to a deep region from the surface layer, the feature which has not been attained by the prior art. For a fixed temperature of the heat treatment, this invention is capable of attaining the extinction of the void type defects to a region of a greater depth than the prior art.
This invention is directed to providing a method of production which enables a silicon semiconductor substrate possessing a defect-free layer of a required depth to be manufactured by performing a heat treatment for a shorter duration than the prior art in forming the defect-free layer in a fixed depth. The objects mentioned above are accomplished by the following items (1)xcx9c(10).
(1) A silicon semiconductor substrate derived from a silicon single crystal grown by the Czochralski method or the magnetic field-applied Czochralski method, characterised by satisfying the relation, 0.2xe2x89xa7V/S/R, providing V denotes the volume of void type defects, S the surface area thereof, and R the radius of spherical defects presumed to have the same volume as the void type defects having the volume of V.
(2) A silicon semiconductor substrate set forth in item (1) above, which contains nitrogen at a concentration of not less than 1xc3x971014 atoms/cm3 and not more than 1xc3x971016 atoms/cm3.
(3) A silicon semiconductor substrate set forth in item (1) above, wherein the void type defects in the silicon semiconductor substrate having an oxygen concentration of not more than 9.5xc3x971017 atoms/cm3 and a nitrogen concentration of not less than 5xc3x971014 atoms/cm3 and not more than 1xc3x971016 atoms/cm3, when presumed to have spherical volumes, have a radius R satisfying Rxe2x89xa630 nm.
(4) A silicon semiconductor substrate set forth in item (1) above, wherein the void type defects in the silicon semiconductor substrate having an oxygen concentration of not more than 8.5xc3x971017 atoms/cm3 and a nitrogen concentration of not less than 5xc3x971014 atoms/cm3 and not more than 1xc3x971016 atoms/cm3, when presumed to have spherical volumes, have a radius R satisfying Rxe2x89xa675 nm.
(5) A method for the production of a silicon semiconductor substrate, characterised by causing a silicon semiconductor substrate set forth in item (1) above as derived from a silicon single crystal grown by the Czochralski method or the magnetic field-applied Czochralski method using a cooling rate of not less than 1xc2x0 C./min while the silicon single crystal being pulled passes through a temperature zone of 1100xc2x0 C. to be heat-treated in a non-oxidising atmosphere at a temperature of not less than 1150xc2x0 C. for not less than one hour at least.
(6) A method for the production of a silicon semiconductor substrate, characterised by causing a silicon semiconductor substrate set forth in item (2) above as derived from a silicon single crystal grown by the Czochralski method or the magnetic field-applied Czochralski method using molten silicon containing nitrogen at a concentration of not less than 1xc3x971017 atoms/cm3 and not more than 1.5xc3x971019 atoms/cm3 and using a cooling rate of not less than 1xc2x0 C./min while the silicon single crystal being pulled passes through a temperature zone of 1100xc2x0 C. to be heat-treated in a non-oxidising atmosphere at a temperature of not less than 1150xc2x0 C. for not less than one hour at least.
(7) A method for the production of a silicon semiconductor substrate, characterised by causing a silicon semiconductor substrate set forth in item (3) above as derived from a silicon single crystal grown by the Czochralski method or the magnetic field-applied Czochralski method using molten silicon containing nitrogen at a concentration of not less than 5xc3x971017 atoms/cm3 and not more than 1.5xc3x971019 atoms/cm3 and using a cooling rate of not less than 5xc2x0 C./min while the silicon single crystal being pulled passes through a temperature zone of 1100xc2x0 C. to be heat-treated in a non-oxidising atmosphere at a temperature of not less than 1150xc2x0 C. for not less than one hour at least.
(8) A method for the production of a silicon semiconductor substrate, characterised by causing a silicon semiconductor substrate set forth in item (4) above as derived from a silicon single crystal grown by the Czochralski method or the magnetic field-applied Czochralski method using molten silicon containing nitrogen at a concentration of not less than 1xc3x97108 atoms/cm3 and not more than 1.5xc3x971019 atoms/cm3 and using a cooling rate of not less than 1xc2x0 C./min and less than 5xc2x0 C./min while the silicon single crystal being pulled passes through a temperature zone of 1100xc2x0 C. to be heat-treated in a non-oxidising atmosphere at a temperature of not less than 1150xc2x0 C. for not less than one hour at least.
(9) A method for the production of a silicon semiconductor substrate, characterised by heat-treating a silicon semiconductor substrate set forth in item (3) above in a non-oxidising atmosphere at a temperature of not less than 1200xc2x0 C. thereby causing the semiconductor substrate to acquire an oxygen concentration of not more than 6xc3x971016 atoms/cm3 at a depth of 1 xcexcm from the surface at the centre thereof, a locally concentrated part originating in a nitrogen segregation exhibiting a concentration of not less than twice as high as the average signal strength at the centre of the silicon substrate depth in the determination of nitrogen concentration by the method of secondary ion mass analysis (SIMS), a surface defect-free layer of a depth of not less than 5 xcexcm at least and less than 12 xcexcm from the surface, and an oxygen precipitate density of not less than 1xc3x97109 pieces/cm3 at the centre of depth of the central part of the silicon substrate.
(10) A method for the production of a silicon semiconductor substrate, characterised by heat-treating a silicon semiconductor substrate set forth in item (4) above in a non-oxidising atmosphere at a temperature of not less than 1200xc2x0 C. thereby causing the semiconductor substrate to acquire an oxygen concentration of not more than 5xc3x971016 atoms/cm3 at a depth of 1 xcexcm from the surface at the centre thereof, a locally concentrated part originating in a nitrogen segregation exhibiting a concentration of not less than twice as high as the average signal strength at the centre of the silicon substrate depth in the determination of nitrogen concentration by the method of secondary ion mass analysis (SIMS), a surface defect-free layer of a depth of not less than 5 xcexcm at least and less than 12 xcexcm from the surface, and an oxygen precipitate density of not less than 5xc3x97108 pieces/cm3 at the centre of depth of the central part of the silicon substrate.
The silicon semiconductor substrate according to this invention is obtained by slicing a segment of a prescribed thickness from a silicon single crystal grown by the Czochralski method (hereinafter referred to as xe2x80x9cCZ methodxe2x80x9d) or the magnetic field-applied Czochralski method (hereinafter referred to as xe2x80x9cmagnetic field-applied CZ methodxe2x80x9d).
To be specific, the CZ method comprises growing a silicon single crystal bar of a required diameter by causing a seed crystal to contact the melt of a polycrystalline silicon raw material accommodated in a quartz crucible and slowly pulling it upward as kept in rotation meanwhile. The crystal while being pulled can be doped with nitrogen by having a nitride placed in advance in the quartz crucible or injecting the nitride into the molten silicon or having the ambient gas encircling the site of crystal growth contain nitrogen from the beginning. In this case, the amount of the dopant incorporated into the crystal can be controlled by adjusting the amount of the nitride, the concentration of the nitrogen gas, or the duration of the injection. The magnetic field-applied CZ method is identical with the CZ method, except it performs the operation of pulling a crystal bar while continuing application of a magnetic field to the interior of the quartz crucible. In this manner, the nitrogen concentration can be easily controlled in the range of 1xc3x971017xcx9c1.5xc3x971019 atoms/cm3 in the silicon melt or in the range of 1xc3x971014xcx9c1xc3x971016 atoms/cm3 in the silicon single crystal.
Further, this invention prefers the oxygen concentration in the silicon single crystal bar to be controlled in the range of 6xc3x971017xcx9c1xc3x971018 atoms/cm3 during the growth of the single crystal bar doped with nitrogen by the CZ method or the magnetic field-applied CZ method.
During the growth of the silicon single crystal bar, the necessity for decreasing the oxygen concentration in this crystal bar to a level in the range mentioned above may be fulfilled by any of the methods heretofore adopted popularly. The adjustment of the oxygen concentration in the range mentioned above can be easily attained by such means as, for example, decreasing the rotational frequency of the crucible, increasing the flow rate of the gas to be introduced, lowering the atmospheric pressure, and adjusting the temperature distribution and the convection in the molten silicon.
Further, this invention, while growing the silicon single crystal bar doped with nitrogen by the CZ method or the magnetic field-applied CZ method, prefers the cooling rate of the growing crystal to be controlled in the range of 1xcx9c15xc2x0 C./min. Actually, the realisation of these conditions for the production of crystal may be attained, for example, by a method of adjusting the pulling speed of the crystal thereby increasing or decreasing the speed of the growth of the crystal. It may be otherwise attained by installing within the chamber of an apparatus for the production of a silicon single crystal by the CZ method or the magnetic field-applied CZ method a device which is capable of cooling the crystal at an arbitrary cooling rate. As the cooling device of this nature, it suffices to use a device which can cool a given crystal by blowing a cooling gas or adopt a method for disposing at a fixed position on the surface of the molten silicon a water-cooling ring adapted to encircle the crystal in growth. In this case, by using the cooling method mentioned above and adjusting the speed of pulling the crystal in growth, it is made possible to confine the cooling rate within the range mentioned above.
Thus, in the CZ method or the magnetic field-applied CZ method, the silicon single crystal bar doped with nitrogen in a required concentration, made to contain oxygen in a required concentration, and grown at a required cooling rate can be obtained. This silicon single crystal bar is then sliced with such a cutting device as the inner peripheral blade slicer or the wire saw and subjected to the steps of chamfering, lapping, etching, and grinding to manufacture a silicon single crystal wafer. Naturally, these steps are enumerated here for the exclusive purpose of illustration. Various other steps such as washing are also conceivable. The sequence of these steps may be altered or part of the steps may be omitted properly to suit the purpose of use of the finished product.
The silicon single crystal wafer which is obtained as described above permits manufacture of a silicon semiconductor substrate which satisfies the relation, 0.2xe2x89xa7V/S/R, providing V denotes the volume of void type defects (i.e. xe2x80x9cempty holexe2x80x9d type defectsxe2x80x9d), S the surface area thereof, and R the radius of spherical defects presumed to have the same volume as the void type defects having the volume of V. Subsequently by subjecting the silicon semiconductor substrate to a gettering heat treatment and/or a heat treatment required for the production of a device, it is made possible to decrease the void type defects in the surface layer with excellent operational efficiency to such an extent as to preclude occurrence of defects in the produced device.
This invention defines the volume of the void type defects. The volume, V, and the surface area, S, were the numerical values obtained in the relevant experiment by tilting a sample of TEM and three-dimensionally measuring the void type defects in shape. The radius, R, was found by calculating the relational expression, V=4ΠR3/3, wherein V denotes the volume found by the visual observation of TEM.
The silicon semiconductor substrate contemplated by this invention has the nitrogen concentration thereof fall in the range of 1xc3x971014 atoms/cm3xcx9c1xc3x971016 atoms/cm3 as determined by the analysis of nitrogen in accordance with the method of secondary ion mass analysis (SIMS). Then, the void type defects in the silicon single crystal, as specifically described herein below, have the shape thereof changed from an octahedron to a plate and further from a plate and/or a plate-like shape of a large ratio of change of shape to an oxygen precipitate (wholly OSF region), depending on the concentration of nitrogen increased by addition and the cooling rate used while the silicon single crystal being pulled passes through a temperature region of 1100xc2x0 C. In this region, since the void type defects of a shape other than the octahedron have a large surface area as compared with the conventional octahedral void type defects, the heat treatment performed subsequently tends to promote diffusion of voids which are point defects and give rise to a surface defect-free layer. As regards the range for the ratio of volume/surface area of the void type defects of the shape of a plate and further the shape of a plate and/or a bar which have a larger surface area than the octahedral void type defects, it suffices to control the void type defects of such a shape as satisfies the relational expression, 0.2xe2x89xa7V/S/R, by the state thereof prior to the heat treatment. When this relational expression is satisfied, the subsequent heat treatment will facilitate the extinction of the void type defects. Then, the amount of added nitrogen necessary for satisfying this condition is required to satisfy at least the condition of this invention, i.e. the range of 1xc3x971014 atoms/cm3xcx9c1xc3x971016 atoms/cm3.
The silicon semiconductor substrate whose void type defects have been rendered more extinctive by the heat treatment is preferred to have an oxygen concentration of not more than 9.5xc3x971017 atoms/cm3 and a nitrogen concentration in the range of 5xc3x971014xcx9c1xc3x971016 atoms/cm3 when the cooling rate is not less than 5xc2x0 C./min before it is heat-treated in a non-oxidising atmosphere such as, for example, an atmosphere of one member or a mixture of two or more members selected from among such gases as hydrogen, nitrogen, argon, and helium. When the cooling rate is not less than 1xc2x0 C./min and less than 5xc2x0 C./min, the oxygen concentration is preferred to be not more than 8.5xc3x971017 atoms/cm3 and a nitrogen concentration to be in the range of 1xc3x971013xcx9c1xc3x971016 atoms/cm3. Then, when the void type defects in the silicon semiconductor substrate whose nitrogen concentration has been controlled as described above have been made to assume spherical volumes, the radius R of such spherical volumes is preferred to satisfy the condition, Rxe2x89xa630 nm, when the cooling rate is not less than 5xc2x0 C./min and the condition, Rxe2x89xa675 nm, when the cooling rate is not less than 1xc2x0 C./min and less than 5xc2x0 C.
Here, the case of using a cooling rate of not less than 1xc2x0 C./min and less than 5xc2x0 C./min and the case of using a cooling rate of not less than 5xc2x0 C./min are discriminated because the nitrogen concentration that induces a change in form of the void type defects from an octahedron through a shape of plate and a shape of bar varies across a cooling rate of about 5xc2x0 C./min as the boundary and the former cooling rate tends to increase the surface area to a greater extent than the latter cooling rate for a fixed amount of added nitrogen and to bring a greater effect in attaining extinction of the void type defects by the subsequent heat treatment even when such void type defects have larger dimensions. The radius R which this invention contemplates in presuming the void type defects to possess spherical volumes, therefore, is allowed to have a larger numerical value in the former case of using a slower cooling rate.
The reason for discriminating the ranges of oxygen concentration by the magnitude of cooling rate is that the ease with which the inner walls of the void type defects grow a silicon oxide film increases in accordance as the cooling rate is decreased and, for the sake of the extinction of the void type defects, the inner wall oxide film must be wholly diffused till extinction as the first step. When the cooling rate is slow, therefore, it becomes necessary to decrease the inner wall oxide film in thickness. To be specific, the oxygen concentration in the silicon semiconductor substrate prior to the heat treatment which is performed for the sake of the extinction of the void type defects must be controlled to a lower level than when the cooling rate is fast. Actually, the oxygen concentration of the silicon semiconductor substrate prior to the heat treatment is preferred to be limited within the range contemplated by this invention as will be specifically described herein below.
Of the silicon semiconductor substrate derived from the silicon single crystal grown by the CZ method or the magnetic field-applied CZ method using a cooling rate exceeding 1xc2x0 C./min, preferably falling in the range of 1xcx9c15xc2x0 C./min, the silicon semiconductor substrate that satisfies the relation, 0.2xe2x89xa7V/S/R, before undergoing the heat treatment contemplated by this invention is heat-treated in the non-oxidising atmosphere mentioned above at a temperature exceeding 1150xc2x0 C., preferably falling in the range of 1150xcx9c1250xc2x0 C., for a duration exceeding one hour, preferably falling in the range of 2xcx9c4 hours when the maximum final temperature is 1150xc2x0 C. or in the range of 1xcx9c2 hours when the maximum final temperature is in the range of 1200xcx9c1250xc2x0 C. to produce a silicon semiconductor substrate which possesses a defect-free layer containing no void type defect in the surface layer and excelling in yield of device.
For the purpose of satisfying the condition, 0.2xe2x89xa7V/S/R, for the shape allowing easy extinction of the void type defects thereby obtaining a silicon semiconductor substrate which has attained extinction of the void type defects in the surface layer of the silicon semiconductor substrate as aimed at by this invention, the silicon semiconductor substrate prior to the heat treatment is only required to contain nitrogen in a concentration in the range of 1xc3x971014xcx9c1xc3x971014 atoms/cm3. For the purpose of enabling the silicon semiconductor substrate to incorporate therein nitrogen in the concentration mentioned above, it suffices to employ the CZ method or the magnetic field-applied CZ method in pulling molten silicon on the condition that the molten silicon contains nitrogen at a concentration in the range of 2xc3x971017xcx9c1.5xc3x971019 atoms/cm3 during the course of a pulling operation. The cooling rate used while the silicon single crystal being pulled passes through a temperature zone of 1100xc2x0 C. is only required to exceed 1xc2x0 C./min. More preferably by defining the nitrogen concentration and the cooling rate respectively, the extinction of the void type defects from the surface of the silicon substrate subsequent to the heat treatment which is aimed at the extinction of the void type defects can be secured to a deeper region. When the cooling rate is not less than 1xc2x0 C./min and less than 5xc2x0 C./min, the nitrogen concentration in the silicon semiconductor substrate is preferred to be in the range of 1xc3x971015xcx9c1xc3x971016 atoms/cm3 and that in the molten silicon in the process of being pulled is preferred to be in the range of 1xc3x971018xc3x971.5xc3x971019 atoms/cm3 for the sake of the incorporation of nitrogen at the concentration mentioned above. When the cooling rate is not less than 5xc2x0 C./min, the nitrogen concentration in the silicon semiconductor substrate is preferred to be in the range of 5xc3x971014xcx9c1xc3x971016 atoms/cm3 and that in the molten silicon in the process of being pulled is preferred to be in the range of 2xc3x971017xcx9c1xc3x971019 atoms/cm3 for the sake of the incorporation of nitrogen at the concentration mentioned above. When the silicon semiconductor substrate derived from the silicon single crystal grown under the conditions mentioned above is heat-treated in the non-oxidising atmosphere mentioned above at a temperature exceeding 1150xc2x0 C., preferably falling in the range of 1150xcx9c1250xc2x0 C. for a period exceeding one hour, preferably falling in the range of 2xcx9c4 hours when the maximum final temperature is 1150xc2x0 C. or in the range of 1xcx9c2 hours when the maximum final temperature is in the range of 1200xcx9c1250xc2x0 C., it produces a semiconductor substrate containing no void type defect in the surface layer and excelling in device yield.
The inner wall oxide film of the void type defects needs control as described above. When the cooling rate during the course of a pulling operation is not less than 1xc2x0 C./min and less than 5xc2x0 C./min, the oxygen concentration prior to the heat treatment is preferred to be not more than 8.5xc3x971017 atoms/cm3. When the cooling rate is not less than 5xc2x0 C./min, the oxygen concentration prior to the heat treatment is preferred to be not more than 9.5xc3x971017 atoms/cm3.
According to this invention, when the silicon semiconductor substrate having the R satisfy the formula Rxe2x89xa630 nm is heat-treated in the non-oxidising atmosphere mentioned above at a temperature exceeding 1200xc2x0 C., preferably falling in the range of 1200xcx9c1250xc2x0 C. to produce a silicon semiconductor substrate having an oxygen concentration of not more than 6xc3x971016 atoms/cm3 at a depth of 1 xcexcm from the surface at the centre of the semiconductor substrate and a locally concentrated part originating in a nitrogen segregation exhibiting a concentration of not less than twice as high as the average signal strength at the centre of the silicon substrate depth in the determination of nitrogen concentration by the method of secondary ion mass analysis (SIMS), the produced silicon semiconductor substrate possesses a surface defect-free layer of a depth of not less than 5 xcexcm at least and less than 12 xcexcm from the surface, enjoys a satisfactory device yield such that the oxygen precipitate density is not less than 1xc3x97109 pieces/cm3 at the centre of depth in the central part of the silicon substrate, and excels in the gettering property.
Further, according to this invention, when the silicon semiconductor substrate having the R satisfy the formula Rxe2x89xa675 nm is heat-treated in the non-oxidising atmosphere mentioned above at a temperature exceeding 1200xc2x0 C., preferably falling in the range of 1200xcx9c1250xc2x0 C. to produce a silicon semiconductor substrate having an oxygen concentration of not more than 5xc3x971016 atoms/cm3 at a depth of 1 xcexcm from the surface at the centre of the semiconductor substrate and a locally concentrated part originating in a nitrogen segregation exhibiting a concentration of not less than twice as high as the average signal strength at the centre of the silicon substrate depth in the determination of nitrogen concentration by the method of secondary ion mass analysis (SIMS), the produced silicon semiconductor substrate possesses a surface defect-free layer of a depth of not less than 5 xcexcm at least and less than 12 xcexcm from the surface, enjoys a satisfactory device yield such that the oxygen precipitate density is not less than 5xc3x97108 pieces/cm3 at the centre of depth in the central part of the silicon substrate, and excels in the gettering property.